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The environment near the summit of Llullaillaco 鈥� a massive Andean volcano straddling the border of Argentina and Chile 鈥� is hostile to animal life: perpetually frigid, exceptionally dry and oxygen poor. But scientists found mice there in 2020, and their latest research adds to evidence that the tiny rodents may actually make the highest reaches of the volcanic peak their home.

A couple of years ago, at the University of Nebraska in Lincoln聽and聽聽at the Austral University of Chile in Valdivia, along with other researchers, reported a new high-elevation record holder among mammals: a leaf-eared mouse now identified as belonging to the species Phyllotis vaccarum, captured atop Llullaillaco鈥檚 summit 6739-metres above sea level.

The discovery was the result of years of repeated expeditions to Llullaillaco, where the rodents had been anecdotally observed on the peak鈥檚 highest reaches by mountaineers.

However, there were still questions about whether the mice really spent their lives near the summit. Most documentation of altitude records for animals are based on single observations, says at Florida State University, one of the researchers.

鈥淚t doesn鈥檛 really tell you if [the animals are] adapted to that extreme elevation,鈥� he says. Individuals may visit high elevations briefly, while the population mostly lives and reproduces at a much lower altitude.

Now, the team has pulled together more data to make the case that the mice also sustain populations at 鈥� or at least very near 鈥� the summit. This includes formally assessing data they had collected between 2011 and 2016, when they found more mice slightly further down the flanks of the volcano, about 6200 metres above sea level.

The researchers also analysed the microbial community collected from soil in a mouse runway 鈥� a route that the mice repeatedly scurry across 鈥� at an altitude of 6154 metres. They discovered it was dominated by animal-associated bacteria. Elsewhere on Llullaillaco, the soil at this elevation mostly contains stress-tolerant fungi instead.

鈥淚t鈥檚 probably not just one mouse that happened to come and defecate, but an accumulation of faecal material and sloughed animal skin to make this radical change,鈥� says Steppan. This adds to evidence that the mice really do spend their lives near the summit.

The researchers believe the mice are record holders among high-living mammals. The closest contenders 鈥� animals like pikas and yaks 鈥� probably don鈥檛 form populations above 6100 metres.

鈥淭he fact that [the mice] can have this self-sustaining population, theoretically reproducing, living their entire lifespan in this extreme environment is pretty incredible,鈥� says at McMaster University in Canada. Low oxygen can make carrying a pregnancy to term difficult and hinder the growth of the young, she adds.

What makes the discovery even more astonishing is that other populations of Phyllotis vaccarum live near sea level 鈥� which means the species has the largest elevation range of any mammal.

It isn鈥檛 known what the mice eat to fuel their cold-exposed bodies, since no vascular plants or insects live near the summit. They may eat bits of vegetation blown from lower elevations, says at California State University in Fresno. Or they could rely on fumaroles 鈥� volcanic vents that produce a warm, wet breeze that can support mossy, mini ecosystems.

Storz and his team are currently investigating one mouse鈥檚 stomach contents to get answers.聽Additionally, Storz and Quiroga-Carmona 鈥� with聽, also at聽the Austral University of Chile 鈥� are studying the physiology and genetics of leaf-eared mice across their elevation range to understand the evolution of their record-setting altitude tolerance.

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